1. Technical Field
Aspects and embodiment disclosed herein relate to a gas scouring apparatus and, more particularly, to a gas scouring apparatus for use in a fluid filtration system to inhibit fouling of immersed fluid filtration membranes.
2. Discussion of Related Art
The importance of filtration membranes for treatment of wastewater is growing rapidly. It is now well known that membrane processes can be used in an effective tertiary treatment system for wastewater including, for example, sewage and provide quality effluent. However, the capital and operating costs of such systems may in some instances be prohibitive. With the arrival of submerged membrane processes where the filtration membrane modules are immersed in a large feed tank and filtrate is collected through suction applied to the filtrate side of the membrane or through gravity feed, membrane bioreactors combining biological and physical processes in one stage promise to be more compact, efficient and economic. Due to their versatility, the size of membrane bioreactors can range from household (such as septic tank systems) to sizes appropriate for use in community and large-scale sewage treatment.
The success of a membrane filtration process largely depends on employing an effective and efficient membrane cleaning method. Commonly used physical cleaning methods include backwash (backpulse, backflush) using a liquid permeate, a gas, or combination thereof, and/or membrane surface scrubbing or scouring using a gas in the form of bubbles in a liquid. Typically, in gas scouring systems, a gas is injected, usually by means of a blower, into a liquid system where a membrane module is submerged to form gas bubbles. The bubbles so formed then travel upwards to scrub surfaces of the filtration membranes in the membrane module to remove fouling substances formed or deposited on the membrane surfaces. The shear force produced largely relies on the initial gas bubble velocity, bubble size, and the resultant of forces applied to the bubbles. The fluid transfer in this approach is limited to the effectiveness of the gas lifting mechanism. To enhance the scrubbing effect, more gas may be supplied. In some instances, this method consumes large amounts of energy. Moreover, in an environment of high concentration of solids, the to gas distribution system may gradually become blocked by dehydrated solids or simply be blocked when the gas flow accidentally ceases.
Furthermore, in an environment of high concentration of solids, the solid concentration polarization near the membrane surfaces may become significant during filtration where clean filtrate passes through membranes and a higher solid-content retentate is left, leading to an increased membrane resistance. Some of these problems have been addressed by the use of two-phase flow to clean the membranes.
Intermittent or cyclic aeration systems which provide gas bubbles on a cyclic basis are claimed to reduce energy consumption while still providing sufficient gas to effectively scrub the membrane surfaces. To provide for such intermittent operation, such systems normally require complex valve arrangements and control devices which tend to increase initial system cost and ongoing maintenance costs of the complex valve and switching arrangements required.
Known intermittent or cyclic aeration systems exist wherein the scouring process operates without complex valve switching. For example, Australian patent application number 200263139 describes such a system. The operation of such a system, however, is hampered when bubble distribution is uneven or when the bubbles do not scour the surfaces of all membranes in a membrane module.